The D/H Ratio in Methane in Titan: Origin and History

Abstract

We propose a new interpretation of the D/H ratio in CH 4 observed in the atmosphere of Titan. Using a turbulent evolutionary model of the subnebula of Saturn (O. Mousis et al. 2002, Icarus 156, 162–175), we show that in contrast to the current scenario, the deuterium enrichment with respect to the solar value observed in Titan cannot have occurred in the subnebula. Instead, we argue that values of the D/H ratio measured in Titan were obtained in the cooling solar nebula by isotopic thermal exchange of hydrogen with CH 3D originating from interstellar methane D-enriched ices that vaporized in the nebula. The rate of the isotopic exchange decreased with temperature and became fully inhibited around 200 K. Methane was subsequently trapped in crystalline ices around 10 AU in the form of clathrate hydrates formed at 60 K, and incorporated into planetesimals that formed the core of Titan. The nitrogen–methane atmosphere was subsequently outgassed from the decomposition of the hydrates (Mousis et al. 2002). By use of a turbulent evolutionary model of the solar nebula (O. Mousis et al. 2000, Icarus 148, 513–525), we have reconstructed the entire story of D/H in CH 4, from its high value in the early solar nebula (acquired in the presolar cloud) down to the value measured in Titan's atmosphere today. Considering the two last determinations of the D/H ratio in Titan—D/H=(7.75±2.25)×10 −5 obtained from ground-based observations (Orton 1992, In: Symposium on Titan, ESA SP-338, pp. 81–85), and D/H=(8.75 +3.25 −2.25)×10 −5, obtained from ISO observations (Coustenis et al. 2002, submitted for publication)—we inferred an upper limit of the D/H ratio in methane in the early outer solar nebula of about 3×10 −4. Our approach is consistent with the scenario advocated by several authors in which the atmospheric methane of Titan is continuously replenished from a reservoir of clathrate hydrates of CH 4 at high pressures, located in the interior of Titan. If this scenario is correct, observations of the satellite to be performed by the radar, the imaging system, and other remote sensing instruments aboard the spacecraft of the Cassini–Huygens mission from 2004 to 2008 should reveal local disruptions of the surface and other signatures of the predicted outgassing.